<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="a1">
<input type="button" value="平方" onclick="f()">
<div></div>
</body>
<script>
    function f() {
        let i = document.querySelector("#a1")
        let d = document.querySelector("div")
        if (isNaN(i.value)){
            d.innerText ="输入错误"
            return
        }
            //当对字符串进行- * / 运算时自动转成数值
        //d.innerText = i.value*i.value;
        //进行加法运算 需要手动将字符串转成数值否则是字符串拼接
        //d.innerText = parseFloat(i.value)+parseFloat(i.value);
        d.innerText = i.value*1+i.value*1;
    }
</script>
</html>